1. Field of the Invention
The present invention relates generally to Micro-Electromechanical System (MEMS) devices, and more particularly but not exclusively to techniques for controlling charge accumulation on surfaces of MEMS or electro-statically actuated devices.
2. Description of the Background Art
In many Micro-Electromechanical System (MEMS) devices, electrostatic actuation is used to move micromechanical structures. An example of a MEMS device that uses electrostatic actuation is the ribbon-type light modulator, such as the Grating Light Valve (GLV™) light modulator commercially available from Silicon Light Machines, Inc., of Sunnyvale, Calif. A ribbon-type light modulator generally includes a number of ribbons, each having a light-reflective surface supported over a substrate by a resilient micro-structure. A ribbon may be deflectable towards the substrate to form an addressable diffraction grating with adjustable diffraction strength. A ribbon may be electro-statically deflected towards the substrate by drive electronics.
One problem frequently encountered with conventional electro-statically operated MEMS is the accumulation or build-up of charge on dielectric surfaces of the ribbons or substrate. Such charge build-up occurs when these surfaces are charged by ions driven by electric field on the surface or across the bulk dielectric. As a result, the voltages required to effect actuation will vary over time, dependent on the history of the applied electric field and external environmental conditions, such as temperature and humidity. In addition, it has been found that charge behavior of dielectric structures appears to be strongly dependent on surface conditions. Dielectric surface charge build-up is therefore difficult to gauge, leading to operational conditions that change over time to the extent that controlled operation becomes difficult if not impossible.
Prior art approaches to charge mitigation and prevention of build-up generally consist of a long bake-out period of the MEMS device in dry nitrogen, with alternate purge and bake periods, to drive off the most likely charge carrier—water. The bake-out is followed by subsequent hermetic sealing of the MEMS device. Several embodiments of this approach have been described in, for example, commonly-assigned U.S. Pat. Nos. 6,660,552, and 6,387,723.
Although an improvement over previous approaches, the bake-out approach has not been wholly satisfactory for a number of reasons. First, baking water from the surfaces of the MEMS device is a reversible process, which means that all presence of water should be prevented over the device lifetime, an impossible condition to achieve. Second, the lengthy bake-purge cycle extends the production process leading to increased production cost and lower throughput. Finally, the bake cycle itself can lead to complications, such as non-uniformity of device properties due to non-uniform heating, thereby reducing production yield. Baking at temperature levels acceptable to the device only partially drives off water, leaving residual amounts. Performance testing of sealed devices shows that initially surface passivation appears to slow charge rates, but over time, charging appears to increase, possibly due to a gradual increase in surface adsorbed water emerging from baked-out surfaces, until an equilibrium is reached.
In addition to the above, prior art approaches require hermetic seals, which can pose further difficulties or problems and increase production cost and lower yield.
Accordingly, there is a need to control the accumulation or build-up of charge on dielectric surfaces, such that electrostatic forces on the ribbons or movable members are determined solely by the voltages applied to the electrodes. It is desirable that the reduction of charge build-up is sufficient to enable more accurate control of MEMS devices, even those exposed to ambient conditions during operation. It is further desirable that the reduction of charge build-up is sufficient to avoid stability issues and prevent potential catastrophic charge build-up (e.g. snap-down).